Mammalian L1 elements (LINE-1) replicate by copying their RNA transcripts into DNA which is then integrated into the genome. In the last 15 million years this process has generated ~550,000 kb of L1 DNA in murine genomes, ~ 20% of the total. In humans L1 retrotransposition causes up to 0.2% of the genetic defects. The ~7 kb L1 element has four regions: a 5' untranslated region, (UTR); two open reading frames (ORFs I and II); a 3' UTR. The 5' UTR has a regulatory function, the ORF I protein binds RNA, the ORF II protein is a reverse transcriptase and endonuclease, and the 3' UTR forms complex intrastrand DNA and RNA structures. L1 elements evolve rapidly and a large number of both past and present replicatively successful L1 subfamilies now exist. This complex subfamily structure of L1 DNA permits a comparative approach to the functional analysis of the L1 genome and for determining the natural history of L1 elements. To examine the molecular basis of both L1 retrotransposition and some of the evolutionary mechanisms of L1 elements we developed an L1 retrotransposition assay in yeast. The lack of endogenous L1 elements in yeast will allow us to unambiguously test if reverse transcriptase switching between RNA templates accounts for replication-dependent recombination. Such recombination could explain both our finding that ancestral L1 sequences are recycled to create novel modern L1 elements, and the repeated acquisition by L1 elements of novel regulatory sequences. We found that L1 DNA greatly stimulates retrotransposition of a yeast indicator gene and are now determining the L1 components required for this activity. Although different rat L1 5' UTRs stimulate gene activity to different extents, the same amount of gene mRNA is found in all cases. Thus a post transcriptional mechanism apparently is involved in L1 regulation. Electroporation of transcripts into cells confirmed this conclusion and showed that L1 5' UTR does not act as an internal ribosome entry site (IRES). We also found that L1 transcripts are initiated well 5' of the 5' UTR. We are now determining the molecular basis of both the L1 influenced transcriptional initiation and the L1 post transcriptional mechanism. Our work on the evolutionary dynamics of L1 elements in both murine rodents and primates produced several major advances. We unambiguously determined and dated speciation events in rodents using L1 subfamilies of known age and found that murines have undergone several intense speciation episodes, the latest of which may be still underway. We also found that novel L1 subfamilies can amplify very rapidly in present day animals and in humans found distinct, human population-specific L1 subfamilies. While determining how these human L1 subfamilies became amplified we found that L1 transposition is a very frequent cause of polymorphism in humans. We are now refining our methods to isolate and characterize all of the L1 related polymorphisms. This will dramatically increase the repertoire of human genetic markers which will be invaluable for both mapping and linkage analysis, and population genetics.